Organic Light Emitting Diode (OLED) devices include a thin film of electroluminescent organic material sandwiched between a cathode and an anode, with one or both of these electrodes being a transparent conductor. When a voltage is applied across the device, electrons and holes are injected from their respective electrodes and recombine in the electroluminescent organic material through the intermediate formation of emissive excitons.
Emissive displays such as OLEDs commonly use anti-reflection films such as circular polarizers to reduce reflection from ambient light caused by the metallic layers of the OLED. A circular polarizer comprised of a linear absorbing polarizer and a ¼ wave film extinguishes a large amount of ambient light incident on the display.
The display brightness is a key attribute that bears a cost in the expense of electronic drive capacity and its associated bulk as well as the emitter lifetime. In addition, the display power efficiency is an important consumer regulatory counterbalance to display brightness.
In OLED devices, over 70% of the generated light is typically lost due to processes within the device structure. The trapping of light at the interfaces between the higher index organic and Indium Tin Oxide (ITO) layers and the lower index substrate layers is a cause of this poor extraction efficiency. Only a relatively small amount of the emitted light emerges through the transparent electrode as “useful” light. The majority of the light undergoes internal reflections, resulting in light being emitted from the edge of the device or trapped within the device and eventually being lost to absorption within the device after making repeated passes.
Light extraction films use internal nanostructures to avoid waveguiding losses within the device. While providing strong light extraction, internal nanostructures, including regular features such as photonic crystals or linear gratings or random features such as nanoparticles, tend to affect ambient contrast defined by a circular polarizer, which may not be desirable in final applications. In order to improve compatibility with a circular polarizer, it has been proposed to use low pitch nanostructures, for example, pitches between 200 nm and 380 nm as described in U.S. Pat. App. Pub. No. 2010/0289038. Alternatively it has been proposed to design OLED pixels such that nanostructures are located outside the emissive area of the subpixel as described, for example, in Japanese Pat. App. Pub. No. 2010272465. However, such approaches reduce the effectiveness of the extraction nanostructure. Thus, a need exists for a light extraction film that simultaneously enhances the efficiency of light extraction via the nanostructures, while also preserving light polarization for reflection extinction with a circular polarizer.